This invention relates to apparatus for the offshore production of hydrocarbon fluids and more particularly but not exclusively is concerned with such apparatus capable of operating unmanned and in a variety of sea depths including relatively deep operation.
According to one aspect of the present invention, there is provided an apparatus for offshore production of hydrocarbon fluids, comprising a base and a buoyant structure secured to said base and connected or adapted to be connected to a subsea well or wells, characterised in that: (A) the apparatus includes storage facility means for fluid(s) received from the subsea well or wells and/or for processed fluid(s) produced by the apparatus; (B) said buoyant structure is tethered to said base by a plurality of flexible tethers; and (C) said buoyant structure comprises (i) a sub-surface housing containing facilities for processing fluids received from the subsea well or wells via said storage facility in said base; and (ii) a column extending upwardly from said housing to a level above the sea surface.
The storage facility means is advantageously in the form of a storage vessel within the base. Alternatively, ancillary buoyant tethered storage units may be employed. The base may be a gravity structure or a piled structure. The buoyant structure may be connected directly to a wellhead, or via the base.
In another aspect, the invention provides a buoyant structure as defined above, the buoyant structure being adapted for connection to a base located on the sea bed and for processing of hydrocarbon fluids supplied to it from a subsea well or wells.
Preferably the main body of the submerged housing comprises a cylindrical section located between upper and lower conical body sections.
In a third aspect, the invention provides a buoyant structure for use in processing oil from a subsea well, which is characterised in that it comprises (i) a sub-surface housing containing facilities for processing fluids received from the subsea well or wells via said storage facility in said base; and (ii) a column extending upwardly from said housing to a level above the sea surface, and in that said sub-surface housing comprises a cylindrical section located between upper and lower conical body sections.
The buoyant structure of this invention is preferably arranged so that its submerged housing is positioned at a sub-surface depth of from 5 to 30 meters, more preferably 8 to 16 meters.
Advantageously, the buoyant structure will be positioned directly above the base, the latter incorporating a storage facility for hydrocarbon fluids. A connecting riser bundle, preferably in the form of a single common riser, can serve to connect the buoyant structure to the well or wells.
Preferred apparatus in accordance with this invention can be constructed to facilitate the production of hydrocarbon fluids from subsea wells at sub-surface depths typically from 200 to 1000 meters, although embodiments can also operate at depths in excess of 1000 meters.
Preferred apparatus in accordance with this invention can be operated unmanned, i.e., without requiring the presence of personnel on board the structure during normal operation. Typically, personnel will only be required on board the structure during specific periodical operations, for example, in order to replenish consumables and to carry out routine maintenance.
Preferred apparatus in accordance with this invention is suitable for extraction of, for example, oil from small and hitherto uneconomic deposits.
Preferably the buoyant structure is arranged to house all of the equipment needed to process fluids (received from the subsea well via the base) for delivery to a tanker for export from the apparatus.
In one embodiment, the buoyant structure is in the form of a vertically configures spar-type buoy. Such a structure may have a plurality of decks, for example, four, five or six decks.
In a second embodiment, the buoyant structure is in the form of a horizontally configured buoy having fewer decks than the first embodiment just described.